Glucocorticoid hormones have potent anti-inflammatory, cardiovascular and metabolic effects. These include clinically important actions in adipose tissue (promoting obesity, impaired glucose uptake, insulin resistance and hyperlipidaemia), liver (promoting increased glucose production, insulin resistance, and hyperlipidaemia), bone (promoting osteopenia and osteoporosis), the immune system (suppressing inflammatory responses and promoting resolution of inflammation), brain (promoting neurotoxicity, cognitive dysfunction, and mood disturbance), blood vessels (promoting vasoconstriction and hypertension) and kidney (promoting sodium retention and hypertension).
Glucocorticoids are steroids which bind and activate glucocorticoid receptors. Characteristically, these are 21-carbon steroids with 17-hydroxyl, 21-hydroxyl, 11-hydroxyl, 3-keto groups and a double-bond in the 4-5 position. Exemplary glucocorticoids include cortisol (hydrocortisone), dexamethasone, prednisolone, triamcinolone, betamethasone, and beclomethasone.
Glucocorticoid action is mediated largely through activation of glucocorticoid receptors (GR). GR are almost ubiquitously expressed in mammalian cells. Upon binding of glucocorticoid ligand in cytoplasm, these receptors are actively translocated to the nucleus, form heterodimers with other transcription factors and homodimers with each other, and regulate transcription of a wide variety of target genes by binding to GR-response elements in 5′ promoter regions. GR activation can stimulate or inhibit gene transcription.
It has been recognised recently that the biological activity of glucocorticoids is influenced in several ways by their peripheral metabolism. Firstly, enzymes metabolising glucocorticoids influence circulating glucocorticoid concentrations and hence control of ACTH and corticosteroid production. Glucocorticoids are inactivated by a series of enzymes, including 6β-hydroxylase, 20-hydroxysteroid dehydrogenase, 5β-reductase, 5α-reductase and 11β-hydroxysteroid dehydrogenases. A-ring reductases (5β-reductase and 5α-reductase) account for the majority of peripheral cortisol metabolism in humans. They convert cortisol to 5β-dihydrocortisol and 5α-dihydrocortisol which are then rapidly converted in liver to 5β-tetrahydrocortisol and 5α-tetrahydrocortisol by 3α-hydroxysteroid dehydrogenase, with the tetrahydro-metabolites then being subject to conjugation by glucuronyl tranferase and sulphonyl transferase enzymes in liver. For exogenous glucocorticoids, activities of these enzymes influences bioavailability. For endogenous glucocorticoids, cortisol and corticosterone, peripheral metabolic clearance rate (determined by the sum of the activities of these metabolising enzymes and by any additional clearance of glucocorticoid in urine or bile) influences circulating glucocorticoid levels and hence influences central feedback control of the hypothalamic-pituitary-adrenal (HPA) axis. Impaired peripheral metabolic clearance results in negative feedback suppression of the HPA axis to maintain normal cortisol levels. Enhanced metabolic clearance results in activation of the HPA axis, which maintains normal cortisol levels in the circulation but at the expense of higher levels of other ACTH-dependent steroids, such as the adrenal androgens dihydroepiandrosterone, androstenedione and their metabolites. This activation of the HPA axis due to enhanced metabolism of cortisol by 5α-reductase has been invoked as a mechanism for androgen excess (and resulting hirsutism and metabolic derangement) in obesity (Andrew et al 1998) and polycystic ovary syndrome (Stewart et al 1990).
Secondly, activity of glucocorticoid metabolising enzymes also influences intracellular glucocorticoid concentrations, and hence modulates activation of corticosteroid receptors within target tissues. This phenomenon is well described for the role of 11β-hydroxysteroid dehydrogenases (11HSDs)(Stewart & Krozowski 1999; Seckl & Walker 2001). 11HSD type 2 is expressed in a limited range of tissues in humans, including distal nephron, colon, and sweat glands. In these sites it inactivates cortisol by converting it to cortisone, an inert steroid, and hence lowers intracellular cortisol concentrations. Failure of this inactivation, for example in congenital 11HSD2 deficiency or after inhibition of 11HSD2 with glycyrrhetinic acid or carbenoxolone, results in increased intracellular cortisol levels (without any change in circulating cortisol levels which are regulated by the HPA axis, as above) and allows cortisol to bind to mineralocorticoid receptors. Normal activity of 11HSD2 prevents significant occupancy of mineralocorticoid receptors by cortisol, and confers on these receptors their selectivity for aldosterone in vivo (Funder et al 1988; Edwards et al 1988). Conversely, 11HSD type 1 is expressed in other tissues, including liver, adipose tissue, the immune system and central nervous system (Seckl & Walker 2001). Here, 11HSD1 reactivates inert cortisone into cortisol, and hence increases intracellular cortisol concentrations. This reaction is important in maintaining activation of glucocorticoid receptors in these tissues (Walker et al 1995; Kotelevtsev et al 1997; Masuzaki et al 2001).
A third mechanism of influence of metabolic enzymes on steroid action has been described for other members of the steroid-thyroid hormone superfamily. This involves conversion of active steroids into metabolites which are themselves biologically active (Stewart & Sheppard 1992). Examples include conversion of thyroxine into tri-iodothyronine by 5′-monodeiodinases, conversion of testosterone into 5α-dihydrotestosterone by 5α-reductases, conversion of oestrone into oestradiol and androstenedione to testosterone by 17β-hydroxysteroid dehydrogenase, and of androstenedione into oestrone by aromatase.
With regard to glucocorticoids, however, metabolites of endogenous glucocorticoid (cortisol in human; corticosterone in rat and mouse) have been reported to be inert, ie without biological activity (Golf et al 1984). With regard to the metabolites produced by 5α-reductase, the subject of this invention, very little experimentation has tested this assumption. Only 5α-dihydrocorticosterone has been examined, with contradictory results. 5α-Dihydrocorticosterone has been reported to oppose glucocorticoid action, eg by lowering activity of gluconeogenic enzymes in liver (Golf et al 1984), but also to mimic glucocorticoid action in inhibition of neuronal long-term potentiation (Dubrovsky et al 1987). These effects have been interpreted as reflecting membrane effects of 5α-dihydrocorticosterone (Dubrovsky et al 1987), a view which is reinforced by the observation that this steroid does not bind to glucocorticoid receptors, as judged by failure to displace bound dexamethasone (Carlstedt-Duke et al 1977).
In clinical use as anti-inflammatory agents (for example in asthma, inflammatory bowel disease, rheumatoid arthritis, polymyalgia rheumatica etc) glucocorticoids induce adverse effects (such as osteoporosis, obesity, insulin resistance, hyperglycaemia, dyslipidaemia, hypertension, mood and sleep disturbance, cognitive dysfunction) because of their actions mediated by glucocorticoid receptors in most tissues. In addition, excess levels of endogenous glucocorticoids may contribute to similar diseases. To prevent these adverse effects of exogenous and endogenous glucocorticoids, while maintaining beneficial anti-inflammatory effects, requires tissue-specific manipulation of glucocorticoid receptor activation. To date, this has been proposed by manipulating reactivation of glucocorticoids within target tissues which express 11HSD1. For other steroids, preventing conversion of the major hormone into an active metabolite also allows tissue-specific manipulation of hormone action, for example in the use of 5α-reductase inhibitors to prevent conversion of testosterone to 5α-dihydrotestosterone in prostate disease or in the use of aromatase inhibitors to prevent conversion of androgens to oestrogens in breast disease. However, no such active metabolite of cortisol or corticosterone is know, so for glucocorticoids this approach has not been proposed.
Thus, there remains a need in the art to identify methods and compositions suitable for modulating the activity of glucocorticoid receptors for therapeutic and other uses.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.